1. Field of the Invention
This invention relates to electronic signal filters in general and more particularly to a signal filter that has improved isolation between input and output terminals.
2. Description of Related Art
Electrical filters are utilized to transmit desired electrical signals by passing or rejecting selected frequency bands. Filters may generally be grouped as lowpass, highpass, bandpass and bandstop filters depending on their characteristics. A lowpass filter passes low frequency electrical signals and rejects high frequency electrical signals. Conversely, a highpass filter suppresses low frequency electrical signals, while passing high frequency signals. A bandpass filter passes electrical signals in a particular frequency band between two frequency points such as a high and low frequency point. A bandstop filter, however, passes all signals except the signals having frequencies between two selected frequency points.
The performance of a filter can be measured using a network analyzer or may be simulated using computer simulation software. Scattering or S-parameter measurements are used to characterize the isolation of the filter. Scattering parameters are a ratio between the incident, reflected and transmitted waves. Typical S-parameter measurements include S11 and S21 measurements. S11 is defined as the measured reflected signal from the device under test. S21 is defined as the measured transmitted signal that passes through the device under test.
When designing very small filters for use at high frequencies, isolating the input port from the output port is a problem because of the small size of the filter. Current filter designs suffer from poor isolation between input and output ports that limit the performance of the filter.
While various types of electronic filters have been previously been used, they have suffered from poor isolation between the input and output terminals.
A current unmet need exists for an electronic filter with improved isolation and a small package size that can be produced at a low cost.